Stellar wind variability in Cygnus X-1 from high-resolution excess variance spectroscopy with Chandra

Abstract

Stellar winds of massive stars are known to be driven by line absorption of UV photons, a mechanism which is prone to instabilities, causing the wind to be clumpy. The clumpy structure hampers wind mass-loss estimates, limiting our understanding of massive star evolution. The wind structure also impacts accretion in high-mass X-ray binary (HMXB) systems. We analyse the wavelength-dependent variability of X-ray absorption in the wind to study its structure. Such an approach is possible in HMXBs, where the compact object serves as an X-ray backlight. We probe different parts of the wind by analysing data taken at superior and inferior conjunction. We apply excess variance spectroscopy to study the wavelength-dependent soft X-ray variability of the HMXB Cygnus X-1 in the low/hard spectral state. Excess variance spectroscopy quantifies the variability of an object above the statistical noise as a function of wavelength, which allows us to study the variability of individual spectral lines. As one of the first studies, we apply this technique to high-resolution gratings spectra provided by Chandra, accounting for various systematic effects. The frequency dependence is investigated by changing the time binning. The strong orbital phase dependence we observe in the excess variance is consistent with column density variations predicted by a simple model for a clumpy wind. We identify spikes of increased variability with spectral features found by previous spectroscopic analyses of the same data set, most notably from silicon in over-dense clumps in the wind. In the silicon line region, the variability power is redistributed towards lower frequencies, hinting at increased line variability in large clumps. In prospect of the microcalorimetry missions that are scheduled to launch within the next decade, excess variance spectra present a promising approach to constrain the wind structure.

arXiv commentAccepted by A&A

References in corpus (47)

• On characterising the variability properties of X-ray light curves from active galaxies
• Mass loss from hot massive stars
• X-ray Spectral Variability and Rapid Variability of the Soft X-ray Spectrum Seyfert 1 Galaxies Ark 564 and Ton S180
• The Discordance of Mass-Loss Estimates for Galactic O-Type Stars
• The Chandra High Energy Transmission Grating: Design, Fabrication, Ground Calibration and Five Years in Flight
• The Mass of the Black Hole in Cygnus X-1
• Atomic Data for Permitted Resonance Lines of Atoms and Ions from H to Si, and S, Ar, Ca and Fe
• Cygnus X-1 contains a 21-solar mass black hole -- implications for massive star winds
• Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1
• Orbital, precessional and flaring variability of Cygnus X-1
• Chandra observation of the fast X-ray transient IGR J17544-2619: evidence for a neutron star?
• The Reflection Component from Cygnus X-1 in the Soft State Measured by NuSTAR and Suzaku
• The response of relativistic outflowing gas to the inner accretion disk of a black hole
• Alternative explanations for extreme supersolar iron abundances inferred from the energy spectrum of Cygnus X-1
• NuSTAR and Suzaku observations of the hard state in Cygnus X-1: locating the inner accretion disk
• Long Term Variability of Cygnus X-1. IV. Spectral Evolution 1999-2004
• Corona, Jet, and Relativistic Line Models for Suzaku/RXTE/Chandra-HETG Observations of the Cygnus X-1 Hard State
• The Soft State of Cygnus X-1 Observed with NuSTAR: A Variable Corona and a Stable Inner Disk
• Coupling hydrodynamics with comoving frame radiative transfer: I. A unified approach for OB and WR stars
• Clumped stellar winds in supergiant high-mass X-ray binaries: X-ray variability and photoionization
• Chandra X-ray spectroscopy of the focused wind in the Cygnus X-1 system. I. The non-dip spectrum in the low/hard state
• Long term variability of Cygnus X-1 V. State definitions with all sky monitors
• Broad-band spectra of Cyg X-1 and correlations between spectral characteristics
• 2D wind clumping in hot, massive stars from hydrodynamical line-driven instability simulations using a pseudo-planar approach
• Wind Accretion and State Transitions in Cygnus X-1
• Ultrafast outflows disappear in high radiation fields
• Atmospheric NLTE-models for the spectroscopic analysis of blue stars with winds. IV. Porosity in physical and velocity space
• Revealing the ultra-fast outflow in IRAS 13224-3809 through spectral variability
• New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars. I. Method and first results
• Coupling hydrodynamics with comoving frame radiative transfer: II. Stellar wind stratification in the high-mass X-ray binary Vela X-1
• Searching for Ultra-fast Outflows in AGN using Variability Spectra
• Accretion form a clumpy massive-star wind in Supergiant X-ray binaries
• Long term variability of Cygnus X-1: VII. Orbital variability of the focussed wind in Cyg X-1 / HDE 226868 system
• From X-ray dips to eclipse: Witnessing disk reformation in the recurrent nova USco
• Chandra X-Ray Spectroscopy of the Focused Wind in the Cygnus X-1 System. II. The Nondip Spectrum in the Low/Hard State - Modulations with Orbital Phase
• 3D simulations of microquasar jets in clumpy stellar winds
• Wavelength Measurements of K Transitions of Oxygen, Neon, and Magnesium with X-ray Absorption Lines
• Laboratory Measurements of the K-shell transition energies in L-shell ions of Si and S
• Using principal component analysis to understand the variability of PDS 456
• Chandra X-ray spectroscopy of the focused wind in the Cygnus X-1 system III. Dipping in the low/hard state
• Discovery of Two Types of X-ray Dips in Cyg X-1
• Wind inhibition by X-ray irradiation in HMXBs: the influence of clumping and the final X-ray luminosity
• Modelling X-ray RMS spectra I: intrinsically variable AGN
• Nature of the Warm Absorber Outflow in NGC 4051
• X-ray Variations at the Orbital Period from Cygnus X-1 in the High/Soft State
• Color-color diagrams as tools for assessment of the variable absorption in high mass X-ray binaries
• Modelling X-ray RMS spectra II: the ultra-fast outflow of PDS 456

Cited by in corpus (1)

• A hidden quasi-periodic oscillation in Cygnus X-1 revealed by NICER